Antenna feed system

ABSTRACT

An antenna feed system is disclosed which is formed by successively combining a phase transformer element providing no beam waist and a phase transformer element providing beam waist, so that deviation in beam width due to frequency fluctuations is compensated by the combined transformer elements. This apparatus may be used in the input of a reflector antenna system, such as Cassegrain antenna, for example, which is operated at high gain over broad frequency bands.

United States Patent 1191 Mizusawa Oct. 7, 1975 52 US. Cl 343/755; 343/781; 333/98 R 51 Int. cl. HOIQ 19/10 [58] Field of Search 343/753, 754, 755, 781,

[56] References Cited Primary ExaminerEli Lieberman Attorney, Agent, or Firm-Oblon, Fisher, Spivak, McClelland & Maier [5 7 ABSTRACT An antenna feed system is disclosed which is formed by successively combining a phase transformer element providing no beam waist and a phase transformer element providing beam waist, so that deviation in beam-width due to frequency fluctuations is compensated by the combined transformer elements. This apparatus may be used in the input of a reflector antenna system, such as Cassegrain antenna, for example, which is operated at high gain over broad frequency bands.

UNITED STATES PATENTS 6 Claims, 17 Drawing Figures 3,150,371 9/1964 Hafner 343/781 Subreflector Launcher *l Lens #2 Lens F 11 l T 8m cu I I3m I5m US. Patent (M11975 sheath 3,911,440'

US. Patent Oct. 7,1975 Sheet 4 of6 IFIG.IO

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4GHz 66H:

REF REC- "TOR EDGE ww ww RADIUS OF REFLECTOR (m) RADIUS OF REFLECTOR (m) F l G l l REFLECTOR EDGE A RADIUS OF REFLECTOR (m) RADIUS OF REFLECTOR (m) F l G I2 U.S. Patent Oct. 7,1975 Sheet 5 of 6 3,911,440

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|- 30 .J l-LI D! RADIUS r. METER FIG. l3 (0) AMPLITUDE w 30 lJ-l O! a o O -so LU 90 3 POSITION LLI cc 2 RADIUS f. METER US. Patent Oc t. 7,1975 Sheet 6 of 6 3,911,440

ANTENNA FEED SYSTEM REFERENCE TO PRIOR APPLICATION This application is a continuation-in-part of patent application Ser. No. 302,906, filed Nov. 1, 1972, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improvement in beam waveguide type antenna feed systems, for use with Cassegrain antennas, for example.

2. Description of the Prior Art Conventional beam waveguide systems, as illustrated in FIG. 1, generally comprise launchers or horns la, lb, and a series of phase transformers 2 employing individual phase transformer elements such as dielectric lenses 2a, 2b and 2c. The beam waveguide system may be used for transmitting wave signals over long distances by installing many phase transformers in a long distance section. The phase transformer elements 2a, 2b and 2c employed in the system are shaped so as to transmit the wave signals with minimum losses. The waves between the phase transformer elements are in the form of a beam which is focused half way between the two phase transformer elements and which diverges from the focal point, as shown by the dotted line in FIG. I. The relatively narrow central portion of the beam is referred to as the beam waist" When the frequency of the transmitted wave changes in the waveguide, the degree of focusing between two phase transformer elements also changes. That is, the beam width of the transmitted wave shown by the dotted line in FIG. 1 changes depending upon the frequency. Thus, when conventional beam waveguides are used as antenna feed systems, it is difficult to maintain the best antenna reflector characteristics over wide frequency ranges. However, in a feed system for a reflector antenna network having a subreflector 3 and a main reflector I, such as the Cassegrain antenna as shown in FIG. 2, it is desirable to have a constant beam width with respect to frequency. Accordingly, a need exists for an improved antenna feed system.

SUMMARY OF THE INVENTION It is therefore one object of this invention to provide an improved antenna feed system.

Another object of this invention is the provision of an improved beam waveguide type antenna feed system.

Yet another object of this invention is the provision of an improved beam waveguide type antenna feed system having a small beam width deviation over a wide frequency range.

Briefly, these and other objects of the invention are achieved by providing a beam waveguide type antenna feed system which comprises a series of phase transformer elements including a first phase transformer element having no beam waist and a second phase transformer element having beam waist. The change in beam width due to frequency variations caused by the first phase transformer element is compensated for by an opposite change caused by the second phase transformer element.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic view of a conventional beam waveguide;

FIG. 2 is a schematic view of an antenna feed system employing a conventional beam waveguide;

FIG. 3 is a geometrical diagram of a conventional phase transformer showing the diameter of the aperture, distance between the transformers, focal length,

etc.;

FIG. 4 is a schematic view of a phase transformer having no beam waist;

FIG. 5 is a schematic view of a phase transformer having beam waist;

FIG. 6 is a schematic view of one embodiment of an antenna feed system in accordance with this invention;

FIG. 7 is a geometrical diagram showing parameters upon which Er (u, r) depends,

FIG. 8 is a schematic view of the embodiments of FIG. 6, when opeated at 4 GHz and 6 GHz;

FIG. 9 is a schematic view of an antenna feed system which does not utilize applicants invention;

FIG. 10 is a graph showing the coincidence between the 4 GHz and 6 GHZ patterns of the embodiment of FIG. 8;

FIG. 11 is a graph showing the non-coincidence between the 4 GHz and 6 GHz patterns of the FIG. 9 antenna feed system;

FIG. 12 is a schematic view of another embodiment of an antenna feed system in accordance with the invention;

FIGS. 13a and 13b are graphs illustrating the diffraction field of the embodiment of FIG. 12;

FIG. 14 is a schematic view of an embodiment which utilizes reflector type phase transformer elements;

FIG. 15 is a schematic view of a horn type embodiment; and

FIG. 16 is a schematic view of a reflector type embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS u Tsin 9 and D112 l l 8A (1.; 1T

and wherein Da is the diameter of the effective aperture;

A is the wavelength of the transmitted signal 6 is the angle between the center line of the two phase transformer elements (or horns) and the line from the center of the first transformer element (or apex of the aperture of the horn) to an optical point on the next phase transformer element;

Ls is the distance from the phase transformer element (or horn) to the next phase transformer element (or horn);

Lf is the distance between the focal point of the wave front and the center of the phase transformer (or horn).

Note that when the focal point is present in the propagating direction of the radiated wave, the sign is plus, but when it is opposite to the propagating direction, the sign is minus.

Er(u, t) represents the diffraction field at the distance Ls from the apertures of a horn, lens, reflector, etc. Using the coordinates shown in FIG. 7, Er(u,t) may be expressed as follows:

m (1 r 8x 1.; T Lf (6) The diffraction field Er(u,t) is based on the Kirchhoff-Huygens principle. The diffraction field has been defined as follows in Hu, Ming-Kuei, Fresnel Region Fields of Circular Aperture Antennas, Journal of Research of the National Bureau of Standards, Vol. 65D, No. 2, p. 137, Mar. Apr. 1961.

In the equation (7) substituting the following variables, equation (3) can be introduced:

When equation (3) is combined with equation (8), the parameters u and t can be further defined.

FIG. 4 shows an example of phase transformer including elements 2a and 2b, wherein no beam waist is present between the phase transformer elements. The beam is parallel, as shown by the solid line, when geometrical optical theory can be applied. This is, when I in equation (2) is positive and Lf is infinite. However, the beam cannot be truly parallel for finite signal frequencies. Thus, in practice, the beam gradually spreads out as shown by the dotted line in FIG. 4, until it reaches the position of the phase transformer 21). The degree of beam spreading is highly dependent upon decreasing fequency, i.e., increasing wave length or decreasing as defined by equation (2). When the I remains constant, the degree of deviation or beam spreading in geometrical optics is constant.

In FIG. 5, the phase transformers 2a and 2b are shown, wherein a beam waist is provided between the two phase transformer elements. The beam is focused at the focal point F, as shown by the solid line when geometrical-optics can be applied. The beam-width at the phase transfonner element 2b" is the same as at the phase transformer element 2a" when the focal point F is at the center between the two phase transformer elements.

In this case, the sign of Lf is plus, and Lf is shorter than LS. Accordingly, the sign of t is minus. Thus, as shown by the dotted line in FIG. 5, when the frequency is finite, the beam from the phase transformer element 2a gradually spreads out compared to the geometrical or theoretical beam, and forms a beam waist at the point F without focusing to a point (the beam waist is a maximum at the point F). The beam gradually broadens from the point F, but the degree of broadening is less than that of the theoretical geometrical beam, whereby the beam width is narrower than that of the geometrical beam. This is the opposite of the case where l is positive. The degree of decrease in the beam width is highly dependent upon decreasing frequency, i.e., increasing wave length and decreasing t.

In FIG. 6, one embodiment of the antenna feed system of the present invention is shown, wherein the phase transformer element 2a" providing a beam waist is positioned after the phase transformer element 2a providing no beam waist, whereby the beam is transmitted to the subreflector 3 of the Cassegrain antenna. In this system, the absolute value of the parameter 2 of the phase transformer element 2a is selected to be equal to that of the phase transformer element 2a". When the absolute value of t" is equal for both transformer elements, the degree of beam width expansion due to one transformer element is equal to the beam width reduction due to the other transformer element. Thus, when to two types of phase transformer elements are combined as shown in FIG. 6, the width of the beam from the first phase transformer element 2a is increased relative to the geometrical beam, as shown by thedotted line, when it reaches the second phase transformer element 2a". However, the width of the beam at the subreflector 3 coincides with that of the geometrical beam as shown by the solid and dashed lines.

Even if the frequency of the transmitted wave changes, the degree of deviation increasing the beam width is the same as the degree of deviation decreasing the beam width in this system. Accordingly, the beam width can be maintained equal to the geometrical beam width over a wide frequency range.

FIG. 8 shows theembodiment of FIG. 6 when operated at 4 GHz and 6 GHz. The'parameters u and t representing the diffraction fields at lens No. 2 from lens No. 1 are as follows:

at 4 GHz at 6 GHz at 4 GHZ at 6 GHz II II w or:

The diffraction fields of the above-mentioned lens system are shown in FIG. 10. Data is measured at the plane of the subreflector. As shown in FIG. 10, the coincidence between the patterns of 4 GHz and 6 GHz is quite good. This is to be distinguished from the situation shown in FIG. 9 wherein applicants invention is not utilized and wherein the beam width and shape of the fields at the subreflector position vary with the frequency as shown in FIG. 11.

Another embodiment of the invention is shown in FIG. 12. When this system is operated at GHz, the parameters u and I representing the diffraction field at the lens 2a from the lens aperture 2a are as follows:

The parameters u and 1 representing the diffraction field at the subrcflector 3 from the lens aperture 3a" are as follows:

The diffraction field of the lens system shown in FIG. 12 is shown in FIGS. 13:! and 13b. The field at lens 211" spreads in comparison with that of geometrical optics. However, the field at the subreflector agrees almost entirely with that of geometical optics. Thus, the beam width on the subreflector 3' is independent of wavelength over a wide frequency range.

Lens used in accordance with the invention may be designed on the basis of geometrical rays. In the example of FIG. 12, the incident rays to the lens 2a are parallel rays and the passed rays are focused at focal point F. Therefore, the focal length of the lens 2a" is equal to the distance from the lens 2a" to F or 2.5m. In the above described embodiment, the phase transformer elements are described as dielectric lenses. However it is also possible to use reflector type phase transformer elements as an alternative structure. It is also possible to combine a plurality of paired phase transformers of the type shown in FIG. 6. In addition, a suitable device can be constructed using one dielectric lens and one reflector type phase transformer.

An embodiment using reflector type phase transformer elements corresponding to the lens system shown in FIG. 12 is shown in FIG. 14. In this case, the reflectors 4 and 4" are paraboloids. The parameters u and t of a reflector as shown in FIG. 16 can be obtained in a manner similar to those of a lens.

A horn embodiment is shown in FIG. 15. The parameters u and t ofa horn as shown in FIG. 15 can be obtained in a manner similar to those of a lens. The geometrical center F should be used as the phase center in this instance.

As clearly explained above, in accordance with this invention, a beam waveguide type antenna feed system is provided having a small beam width deviation with respect to changing frequency. In the apparatus of the invention, a phase transformer element providing no beam waist and a phase transformer element providing beam waist are paired so that the distortion caused by one element is cancelled by the other. When this system is used as an antenna feed for a Cassegrain antenna, the best performance of the antenna can be realized Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An antenna feed system having minimal beam width deviation as a function of frequency, comprising:

at least two phase modifying means coupled in series,

one of said phase modifying means having a characteristic parameter I defined by the equation:

wherein Da is the diameter of the effective aperture of said one phase modifying means, A is the wavelength of the signal passing through said antenna feed system, Ls is the distance between said "two phase modifying means and Lf is the distance between the focal point of the wave front of said signal and the center of said one phase modifying means; and

the other of said two phase modifying means is characterized by a parameter t, wherein t has the same absolute magnitude as the parameter I, but the opposite polarity.

2. An antenna feed system having minimal beam width deviation as a function of frequency as in claim 1, wherein:

said phase modifying means comprise dielectric lenses. 1

3. An antenna feed system having minimal beam width deviation as a function of frequency as in claim 1, wherein:

said phase modifying means comprise reflector elements.

4. An antenna feed system having a minimal beam width deviation as a function of frequency as in claim 1, wherein:

said one phase modifying means comprises a dielectric lens and said other phase modifying means comprises a reflector element.

5. An antenna feed system having minimal beam width deviation as a function of frequency as in claim 1, wherein:

said phase modifying means supply input signals to an antenna.

6. An antenna feed system having a minimal beam width deviation as a function of frequency as in claim 1, wherein:

said phase modifying means supply input signals to a 

1. An antenna feed system having minimal beam width deviation as a function of frequency, comprising: at least two phase modifying means coupled in series, one of said phase modifying means having a characteristic parameter t defined by the equation:
 2. An antenna feed system having minimal beam width deviation as a function of frequency as in claim 1, wherein: said phase modifying means comprise dielectric lenses.
 3. An antenna feed system having minimal beam width deviation as a function of frequency as in claim 1, wherein: said phase modifying means comprise reflector elements.
 4. An antenna feed system having a minimal beam width deviation as a function of frequency as in claim 1, wherein: said one phase modifying means comprises a dielectric lens and said other phase modifying means comprises a reflector element.
 5. An antenna feed system having minimal beam width deviation as a function of frequency as in claim 1, wherein: said phase modifying meanS supply input signals to an antenna.
 6. An antenna feed system having a minimal beam width deviation as a function of frequency as in claim 1, wherein: said phase modifying means supply input signals to a Cassegrain antenna. 